Functional anatomy of the carpometacarpal joint of the thumb.

The first carpometacarpal joint is formed by the proximal joint facet of the first metacarpal and the distal joint on the trapezium. The morphologic features of these facets, together with a lax but strong joint capsule, give the thumb great mobility and play a major role in the opposition of the thumb. Based on the kinematic behavior of the joint, it was given the functional status of a ball-and-socket joint. To analyze the motion patterns of the first metacarpal with regard to the trapezium, models were proposed based on the principles of regularly curved joint surfaces. However, it has been acknowledged that irregularly curved surfaces attuned to a specifically shaped ligamentous system offer the morphologic basis for understanding mobility at the base of the thumb. The stability of the first carpometacarpal joint is not guaranteed by joint surfaces and ligaments alone because differences in mobility between the anatomic bone-ligament specimen and the joint in vivo are conspicuous. The function of muscles crossing the joint is essential. The close structural relation between the tendon of the abductor pollicis longus and the first carpometacarpal joint was observed. Five distinguishable tendons of this muscle end at different parts of the joint. The interweaving of some of the tendons with the ligamentous system undoubtedly influences the restraining and directing function of the ligamentous system.